This invention relates to recovery of precious metals, e.g. gold and silver, from ores having a sulfidic sulfur and/or elemental sulfur content, such as sulfide-containing leachable ores of the pyritic, arsenopyritic, or arsenian pyrite type, refractory carbonaceous sulfide ores which have been pretreated, ores which are being post-treated, tailings, previously considered waste grade ores (which still have sufficiently high gold and silver content) and overburden ores having fairly low gold content and which may be considered waste ores.
This invention also relates to the recovery of non-precious metal values from ores having a sulfidic and/or elemental sulfur content whether as an incident to the recovery of precious metals or as a recovery of the non-precious metals.
More particularly this invention relates to a specific treatment of particularly prepared ores of vast quantities and typically leached in heaps, dumps, tailing dumps, or waste dumps and the like. Still more particularly this invention relates to an ore treatment which starts with a preparation of particulates of specific design characteristics making the recovery of precious metals in low amounts and/or the recovery of non-precious metals especially attractive and suitable for a heap or dump leaching, a construction of these specifically inoculated particulates and a heap or dump constructed from the specifically inoculated particulates suitable to an outstanding degree for biooxidation reactions with either single, mixed, layered, or staged biooxidant bacteria cultures.
Further, this invention relates to especially suitable form of a biooxidized and treated ore used in subsequent down-stream precious metal extractions such as by thiourea, or, after heap reconstruction, by thiosulfate or cyanide extraction of the precious metal values in the ore heap or in subsequent downstream nonprecious metal extractions, such as by suitable lixiviants, of the desired metal value(s) in the ore heap. Foremost amongst the ores being treated for precious metal recovery are gold ores. Foremost amongst the ores being treated for nonprecious metal recovery are copper, zinc, nickel, molybdenum, cobalt and uranium ores.
Typically precious metal containing ores are leached with cyanide as the most efficient leachant or lixiviant for the recovery of precious metal values from the ore. It would also be highly desirable to recover nonprecious metal values by heap leaching or lixiviation.
However, because of the mineralogy of various ores, access to the precious and/or nonprecious metal in the ore by cyanide or other lixiviant is low for an economical extraction of the precious metal and/or nonprecious metal values in an ore. If the cyanide extraction produces small or negligible amounts of gold, an ore is said to be refractory or highly refractory. Various methods have been employed to increase the extractability of the precious and/or nonprecious metals. A good summary article describing the prior problems is that authored by Kantopoulos et al., Process Options for Refractory Sulfide Gold Ores: Technical, Environmental, and Economic Aspects, Proceedings EPO ""90 Congress, D. R. Gaskell, Editor, The Minerals, Metals and Materials Society, 1990.
A typical component which causes the refractoriness of the ore is predominantly a carbonaceous type component either inorganic or organic. The organic carbonaceous materials are also classified as acid insoluble carbonaceous materials. Gold found in ores dispersed within or occluded in a sulfide matrix may be considered refractory because of inaccessibility of such gold by cyanide leaching. Similarly, nonprecious metal values found in ores either dispersed within or occluded in a sulfide matrix or present as metal sulfides are also not readily recoverable by heap leaching or lixiviation.
When treating such ores, the economic considerations dictate the selection of the process or the pretreatment of the ore to render it amenable first and foremost to cyanide extraction even though other gold lixiviants may be used. Similarly, it is highly desirable with nonprecious metal values in sulfidic ores to render them recoverable by heap leaching or lixiviation.
As one of the desired treatment steps prior to cyanidation or comparable lixiviation, roasting of ores in presence of air is typical. Lately oxygen or oxygen and air roasting, at low temperatures, have showed considerable promise. Other commercial ore treatment methods prior to cyanidation are high pressure oxygen and/or oxygen-ozone pretreatment, chlorine pretreatments, hypochlorite pretreatments and the like.
To improve cyanidation of ores, during such cyanidation ozone, or ozone and oxygen, or oxygen, or a surfactant, or combinations of these are also employed. In the instance of gold recovery, methods such as xe2x80x9ccarbon-in-pulpxe2x80x9d (or xe2x80x9cCIPxe2x80x9d) and xe2x80x9ccarbon-in-leachxe2x80x9d (or xe2x80x9cCILxe2x80x9d) are used to improve cyanidation reactions and gold recovery.
However, cyanidation has certain shortcomings, primarily an ore material must be neutralized after an acid generating treatment as cyanidation must be carried out on the alkaline side of the pH scale; likewise high cyanide consumption renders a process less attractive. When using thiourea, neutralization of the ore is not as demanding and does not affect thiourea extraction of gold, but the extraction economies are impaired by the higher cost of thiourea and the reduced efficiency when compared with cyanide.
Other compounds which have been used and offer promise because of reagent costs are compounds such as thiosulfates of which ammonium thiosulfate is one of the desirable candidates. Although still other materials are used for gold recovery, these are not yet of industrial significance.
When ammonium thiosulfate and the like are used, neutralization of ore is required as appropriate pH ranges are neutral to alkaline, e.g. to about pH 7 to 10 and preferrably to at least about 9. As pyritic sulfidic ores and many other ores need to be neutralized because of the acidity of these ores when subjected to oxygenation or biooxidation and like treatments, separate process steps are required.
Inasmuch as gold is occluded in the sulfide matrix of the ore, the accessibility by cyanide has sought to be improved for these ores; the same is also true when considering an appropriate sulfide, e.g., pyrite for oxidation or biooxidation. Although various oxidation or biooxidation reactions have been tried such as vat, autoclave, slurry or liquid solution oxidations, these reactions are not practical when using large ore bodies having low gold content. As one of the approaches to oxidation of low content metal sulfide ores, biooxidation has come into prominence and much effort has been expended in research. Biooxidation was first applied to copper. Biooxidation of copper ore has been a well tried method although it is considered fairly slow.
When biooxidation is coupled with oxidative bioleaching, i.e. when direct, indirect and even galvanic leaching reactions are involved, some of the disadvantages of the slow biooxidation reactions are mitigated. Biooxidation reactions typically involve arsenopyritic and pyritic iron sulfide-containing ores including those that have some refractory carbon components present. Biooxidation, however, can suffer from inhibitory concentrations of some metals present in the ore. Biocidically active metals are such as arsenic, antimony, cadmium, lead, mercury, molybdenum. Ions such as chlorine, bromine and the like affect the biooxidation processes. Because of slow growth rates for some bacteria as well as temperature variations in a typical ore dump undergoing sulfide oxidation, considerable efforts have been expended to improve the rate constraints which have limited or held back the potentially very useful application of biooxidation.
Hence, considerable investigation has been made of the various limiting conditions concerning commercial biooxidation including such factors as ores in heaps or in slurry form, the use of surfactants, the use of potentiators or biooxidation promoters such as silver, aluminum, etc., appropriate selection and growing of robust bacteria which would be resistent to the inhibitory biocide activity of metals such as arsenic and growing the bacteria in profuse amounts. Other considerations have been such as nutrient access, air access and carbon dioxide access for making the process even more efficient and thus an attractive ore treatment option. References illustrating these efforts are such as by Bartlett, Aeration Pretreatment of Low Grade Refractory Gold Ores, Minerals and Metallurgical Processing, pp 22-29, (February 1990); Bennett et al, Limitations on Pyrite Oxidation Rates in Dumps Set By Air Transport Mechanisms, Biohydrometallurgy, Proceedings of Jackson Hole Symposium, August 13-18, 1989 Canmet (1989); Burbank et al, Biooxidation of Refractory Gold Ore in Heaps, Ch. 16, pp 151-159 in Advances in Gold and Silver Processing, Reno Proceedings of Symposium xe2x80x9cGoldtech 4xe2x80x9d, Reno, Nev., Sep. 10-12, 1990, Society of Mining, Metallurgy and Exploration, Publisher, 1990; Dix, Laboratory Heap Leach Testing: How Small and Large Scale Tests Compare, Mining Engineering, June 1989, Pages 440-442.
Amongst the methods seeking to improve biooxidation many methods have been proposed for mechanically increasing the access of the biooxidant bacteria to the ore, These methods have relied upon agitation of the ore either in tanks, slurries, providing circulation in vessels or reconstitution and remixing of the materials including stirring, raking, forming an improved slurry, transfer of slurry materials, providing stirred tank basins or have addressed various aspects of heap construction and utilization. References to such considerations are found in an article by Andrews, Large-Scale Bioprocessing of Solids, Biotechnology Progress, Vol. 6, pp 225-230, 1990.
Patents which illustrate some of these methods mentioned above are found such as in U.S. Pat. No. 4,324,764 concerning mechanical distribution of ores or distribution of ores by conveyors such as in U.S. Pat. No. 4,571,387 or a change in heap structure such as in U.S. Pat. No. 4,279,868 or stagewise heap formation such as in U.S. Pat. No. 4,017,309; or a stirred tankxe2x80x94semi xe2x80x9cheapxe2x80x9d construction such as disclosed in U.S. Pat. No. 4,968,008.
However, when treating large amounts of waste heap material or tailing material, the normal considerations that are applicable in high grade precious metal ore treatments are not viable. For waste ore treatment, economics often dictate a one-shot type of heap formation, e.g. for the depth, the size, the reactant accessibility, etc. Moreover, for biooxidation, the induction times concerning biooxidants, the growth cycles, the biocide activities, viability of bacteria and the like become important because the variables such as accessibility, particle size, settling, compaction and the like are economically irreversible once a heap has been constructed as such heaps cannot be repaired except on a very limited basis. For example, compaction problems such as are encountered in heap treatment of ores, and others such as puddling, channelling, or nutrient-, carbon dioxide-, or oxygen-starving, uneven biooxidant bacterial distribution, and the like have been addressed in a number of investigations with respect to biooxidation. Such problems are also encountered in cyanide leaching.
For example, to solve channelling in percolation leaching by cyanides it is known to agglomerate the ore materials of high grade ores such as disclosed in U.S. Pat. Nos. 4,256,705 and 4,256,706. Other approaches to improve percolation leaching by cyanides include addition of fines such as flocculating materials, fibers, wood, pulp and the like as disclosed in U.S. Pat. No. 4,557,905. The last patent discloses leachable matrix formation to allow for access of cyanide to the precious metal values.
An ultimate, albeit impractical, suggestion for cyanide leaching has been found in U.S. Pat. No. 4,424,194 which shows making useful articles and then leaching these. This patent may have as its progenitor the early U.S. Pat. No. 588,476 of Aug. 17, 1887, which discloses porous casts made of gold xe2x80x9cslimesxe2x80x9d and gypsum. These casts are thereafter broken and leached.
Although for a variety of different reasons agglomeration has been practiced in the metallurgical arts such as in high temperature blast furnace art for various feed material preparations for blast furnaces, opposite suggestions have also been found concerning non-agglomeration and extraction of metals such as the pulp-liquid extraction described in U.S. Pat. No. 3,949,051. Extraction of the precious metals from heaps, preformation of heaps and heap treatment is found such as in U.S. Pat. No. 4,017,309 and 4,056,261.
Further improvements for access of cyanide to the precious metals have been described in U.S. Pat. Nos. 4,318,892 and 4,279,868 as well as U.S. Pat. No. 4,301,121. All of these attempts have sought to improve the distribution of the leachant or the mixing ratios of the ore to the lixiviant, but these attempts are typically addressed to providing better access for cyanide and to overcome the ostensible refractoriness of the ore. Other like disclosures have been found in U.S. Pat. Nos. 4,324,764 and 4,343,773.
Heap improvements have been found in the construction of the particles such as paste formation with the lixiviant and subsequent ageing of the ore on treatment of the same, described in U.S. Pat. No. 4,374,097. Likewise, specific berm construction for the improved extraction of liquids from a specifically constructed heap has been found in U.S. Pat. No. 4,526,615. Similarly various particle specifications have been described for the ore particle treatment including the micro agglomerates of a size of 500 microns (and lower) found in U.S. Pat. No. 4,585,548.
In all of these heap formations, heap treatments or heap leaching methods, shortcomings have been sought to be overcome by the increase of cyanide efficiency such as by oxygen addition, e.g. in U.S. Pat. No. 4,721,526, or the use of various liquors in the recovery of gold described in U.S. Pat. No. 4,822,413.
Agglomerating agents for copper ores are shown in U.S. Pat. No. 4,875,935. Opening up clogged heaps has also been shown and discussed in U.S. Pat. No. 3,819,797 and heap treatment for distribution of a lixiviant is disclosed in U.S. Pat. No. 5,005,806. Finally, both conjoint crushing and agglomeration of ore has been discussed in U.S. Pat. No. 4,960,461.
The present invention relates to an improvement for the recovery of desired metal value(s) from an ore material comprising those desired metal value(s) and a matrix material having a sulfur content wherein the sulfur is present in an oxidation-reduction state of zero or less, but more typically such as sulfidic and/or elemental sulfur. The process of the present invention comprises
a. optionally adjusting the pH of the ore material to a pH of less than 2.5, separately or conjointly with acid solution caused partial agglomeration,
b. forming particulates from particles of the ore material with an inoculate comprising microbial agent(s) capable of at least partially biooxidizing the sulfur content,
c. forming a heap of said particulates,
d. biooxidizing the sulfur content in the matrix and
e. recovering the desired metal value(s) either from the biooxidizing solution leachant or a specific leachant for the desired metal value.
The present invention further relates to said particulates and to heaps formed of said particulates. The terms xe2x80x9corexe2x80x9d or xe2x80x9core materialxe2x80x9d as used herein includes not only ore per se, but also concentrates, tailings or waste in which sufficient metal values exist to justify recovery of those values.
The desired metal value(s) may be selected from
Group IB metals of the periodic table of elements (CAS i.e. Chemical Abstracts Service version)
(copper, silver, and gold);
Group IIB metals
(particularly zinc);
Group IV A metals
(germanium and lead, particularly lead);
Group VA metals
(particularly arsenic and antimony);
Group VIB metals
(chromium, molybdenum and tungsten, particularly tungsten);
Group VIII metals
(iron, ruthenium, osmium, cobalt, rhodium, iridium, nickel, palladium and platinum, particularly, nickel, palladium and platinum) and
The Actinide series metals
(particularly uranium).
Preferred among these metals are copper, silver, gold, zinc, cobalt, nickel and uranium.
The matrix material, in addition to having the described sulfur content, may comprise one or more inorganic metallosulfur compounds in which the sulfur moiety conforms to the description of the sulfur content and the metal moiety comprises a metal as previously described. Examples are as follows:
In this context, the desired metal value(s) may be present in the ore material either as
1. an elemental metal, such as gold, dispersed within or occluded in the matrix material;
2. a compound, such as a metal oxide, dispersed within or occluded in the matrix or;
3. a component of the matrix material, such as a metal sulfide.
The present invention renders such desired metal values accessible to recovery treatment in the sense that biooxidation of the sulfur content either exposes the elemental metal or compound for further recovery treatment or it also renders the desired metal values soluble or otherwise accessible to recovery treatment.
The present invention has preferable value in the recovery of gold from low gold content gold ore materials having a sulfidic sulfur content and more preferably in those instances where the matrix material comprises an iron-sulfur compound. Of particular interest are low gold content refractory pyritic and arsenopyritic gold ores.
While the following discussion illustrates the application of the present invention to this metal value/matrix material system, it should be understood that the present invention has application to the following metal values:
Group IB metals of the periodic table of elements (CAS version)
(copper, silver, and gold);
Group IIB metals
(particularly zinc);
Group IVA metals
(germanium and lead, particularly lead);
Group VA metals
(particularly arsenic and antimony)
Group VIB metals
(chromium, molybdenum and tungsten, particularly tungsten);
Group VIII metals
(iron, ruthenium, osmium, cobalt, rhodium, iridium, nickel, palladium and platinum, particularly, nickel, palladium and platinum) and
The Actinide series metals
(particularly uranium).
and to matrix materials that may contain these metals as inorganic metallosulfur compounds wherein the sulfur moiety conforms to the foregoing description.
It should be borne in mind in considering a gold metal value/pyritic or arsenopyritic matrix material system that the gold is present in the ore material in its elemental form dispersed within or occluded in the pyrite or arsenopyrite and that biooxidation renders the gold accessible in that the oxidation of the sulfur content and the iron content solubilizes the matrix.
In none of the references described in the Background of the Invention, relating either to the first stage bacterial biooxidation or the coupled second stage downstream cyanide or like extraction has there been a disclosure for forming specific particulates with a bacterial solution as suitable for low metal content ores treated in a heap; as well as for the biooxidation of low content gold ores. By xe2x80x9clowxe2x80x9d it is meant a gold-containing ore of less than about 0.07 oz/ton of ore. (While the present invention will be described in terms of its desirability in recovering gold from such low content gold ores, it should be understood that it has application to the previously described metals in other levels of metal value content.)
By the term xe2x80x9cparticlesxe2x80x9d it is meant the individual particles found in the ore such as run-of-the-mine- ore; further, it is meant, ore particles formed after either primary or secondary grinding or crushing; by the term xe2x80x9cparticulatesxe2x80x9d it is meant the body or shape which is built up from the individual particles properly coated with an inoculant solution(s) containing bacteria; or from sulfuric acid containing solution followed by an inoculant solution containing a specific bacterium or bacteria of a specific undifferentiated strain or a mixture of bacteria. Such particulate design, construction or formation and the concomitant heap construction have especially desirable properties for the reactant, i.e. the biomass distribution, dispersal and access to: a) the metal values in an ore, particularly low-concentration metal values, or b) low sulfide and metal, particularly low-concentration metal values in an ore. None of the references discloses an economically attractive method for improving the proper initial construction of a heap, for biooxidation purposes, as well as to provide for a more efficient method e.g. cyanidation, for precious metal recovery downstream after biooxidation has taken place in the initially, properly constructed heap.
In its essential aspects, the present invention is directed to a biooxidation stage of an ore in the form of particulates with various pre-treatment and post-treatment steps related to the biooxidation. A subsequent precious metal recovery stage for extracting the precious and/or nonprecious metals from the biooxidized ore body is made now more advantageous because of the initial formation of the ore particulates with a bacteria coating which, as a result of improved biooxidation, makes the ores now especially amenable for the subsequent recovery, such as by cyanide or other extraction, of the desired metal value(s) from such biooxidized ores.
In accordance with the invention, a combination of steps is proposed which has interrelated a number of variables and has sought to reduce the rate limiting conditions for metal ore materials, especially for low precious metal content ore materials heretofore considered wastes and heretofore not capable of treatment for recovery of the precious metals therein such as in biooxidation tanks because of the low sulfur content. By low sulfur content it is meant ores of less than about 0.2 to 0.3% sulfide by weight. While the present invention, has been described with respect to low sulfur content with respect to the recovery of precious metal values such as gold, it is also amenable to use with higher sulfur content ores.
Thus, in accordance with the present invention and its preferred mode, waste materials having a gold content as low as 0.07 oz/ton and even as low as 0.02 oz/ton of gold in an ore may now be economically treated for recovery of the precious metal values. A preferred range is for ores of a gold content above 0.02 oz/ton of ore. Of course, the precious metal values, such as gold, in the ores significantly above the indicated threshold values are also recoverable but other means or options provide fairly attractive and economically competing alternatives which make the present process only a matter of choice in circumstances where such choice needs to be made.
Thus, it is considered practical in accordance with the present invention to treat tailings, waste material, or low grade overburden or previously exhausted dump material if the precious metal values in such ore bodies are at or above 0.02 ounces of gold per ton of ore or the monetary equivalent thereof in case of gold and silver or silver alone.
Still further, it has been found that the discovery, as disclosed herein, makes the process especially amenable to the low grade ores as a heap or dump treatment, with outstanding recovery rates for the metal values in the ores which are subjected to the herein disclosed method.
For example, the present invention contemplates the proper agglomeration of a distribution of fine and coarse ore material with the concomitant proper distribution of the biooxidant bacteria.
It is to be understood that within the body defined by the heap, particulates of different size characteristics and distribution are also contemplated as being an attractive feature of this invention, especially to provide for access to regions previously considered xe2x80x9cdeadxe2x80x9d regions in a heap and now made accessible by the present process.
The above ore material particulates when properly formed and distributed in a heap provide the advantages because the inoculant biooxidant material is appropriately made to contact the individual particles and/or the smaller particles are appropriately layered on the larger particles to form the particulates. It is to be noted that agglomeration is only one of the methods for proper particulate formation and other equivalent methods shall become evident from the description of the desired particulates. Within such formed particulates or layers thereon are found the well established flora across the spectrum of the materials and thus the fine ore material particles provide an extremely attractive distribution of bioreactant bacteria over the entire ore body in a heap as defined by the various particulates formed also from unwanted, very small size particles called xe2x80x9cslimesxe2x80x9d in admixture with larger particles. The same and different bacteria may now be introduced in an appropriate location if dictated by temperature, pH, biocide, access, water drainage (i.e. water saturation) and like considerations. Further, this invention also concerns itself with proper particulate formation by agglomeration from crushed ores which are being heaped on a dump heap at the same time as these particulates are treated with an inoculant liquor containing the suitable bacteria for particular layers in a heap.
Other and various inoculation/particulate formation features of this invention will be disclosed in conjunction with the formation of the heap and the distribution of the biooxidant bacteria throughout the ore mass and the heap.
In connection with the above, pretreatment steps may be practiced such as acid treatment with sulfuric acid to neutralize or partially neutralize high acid consuming ores (thereby partially also agglomerating the ores), followed by the further inoculation and particulate formation and distribution of the biooxidant bacteria throughout the ore mass as it is being treated for deposition on a heap.
In accordance with the invention, the surface covering of the particulate with and the biooxidant bacteria distribution throughout are highly desirous. These features provide for an extremely advantageous access of the bacteria to the sulfide matrix in the ore in various size particles of the ore as well as for the rapid growth and multiplication of the bacteria. The preferential attack on pyrites, such as arsenic pyrites (arsenopyrites and/or arsenian pyrites) and the tailoring and design of subsequent or supplemental biooxidant leachant solutions now give a number of options to devise an optimum treatment for a particular ore.
Because of the possibility to have mixed biooxidant bacterial ore particulates in a heap, the ability to layer a heap, and to add various types of bacteria at various points in the particulate formation, the process provides for flexible and tailor made heap constructions. This flexibility also holds true for layering sequentially or conjointly with mutually compatible bacteria, thus offering different advantages and capabilities with respect to the ore material being treated, e.g. consortia for attacking other components in the ore, e.g. acid insoluble carbon. In this regard, reference should be made to the microbial agents and their method of use described in U.S. patent application Ser. No. 07/750,444, filed Aug. 20, 1991, now U.S. Pat. No. 5,127,942, granted Jul. 7, 1992; the disclosure of which is incorporated herein by reference.
Thus various bacteria that have outstanding characteristics for treating ore materials containing biocidally active metals, such as arsenic, antimony, cadmium and the like, in large quantities in conjunction with iron which is present, have provided for an advantageous intermixing of various cultures and at various points in the treatment cycle and in the heap body.
As a consequence of these advantages, short induction periods for growth have been experienced, better air and carbon dioxide accessibility provided, better nutrient infusion or supplementation practiced, more bio-mass is maintained throughout the heap, compacting and/or clogging are minimized, outstanding permeability is achieved, better percolation is achieved, puddling and/or channelling are minimized, water logging is avoided etc., etc. These and other advantageous features make the process very attractive due to the novel particulate design and heap construction resulting in an outstanding method for recovery of desired metals from metal ores, particularly precious metals from low content precious metal ores.
As one of the outstanding advantages of this invention, the first stage treatment steps have provided for down stream advantages for the recovery of the desired metal values, such as by a second stage, i.e. the cyanide or other lixiviant treatment for the extraction of the metal values or by recovery directly from the bioleaching solution. These second stage advantages result from inter alia, improved porosity associated and achieved with bioleaching, good permeation of lixiviant, etc. Other advantages are such as: relatively fast rate of extraction; a heap treated ore of outstanding accessibility to the cyanide material or other lixiviant or the bioleaching solution itself; reduced cyanide/lixiviant/bioleachant consumption; and other heap type advantages, e.g. regeneration of the lixiviant; flexibility to meet variations in the treatment; an ability to neutralize more easily a heap (if such is desired); the reduction of the neutralization requirements because of the attractive wash cycles (which have been found to exist as a result of the first stage heap treatment with the biooxidant material); and, other and further advantages mentioned herein.
Consequently, as one of the features of this invention, the reconstitution of the heap for cyanide treatment or other lixiviation has become economically attractive with the dispersed porous biooxidized matrix allowing for washing and attractive neutralization of the acidic heap material for the subsequent cyanidation or other lixiviation of the ore.
These and other advantages have further manifested themselves e.g. in that cyanocide fungi and other cyanide degrading microorganisms can now be readily used for the post treatment of the exhausted heap such as found in the disclosure in U.S. Pat. No. 4,402,831. The improved intra particle and inter particle accessibility allows cyanide elimination.